The invention relates to a nuclear reactor vessel and more particularly to a nuclear reactor adapted to reduce the rate of degradation of structural members in its core region.
A pressurized water nuclear reactor vessel in a commercial electric power generating plant recirculates an aqueous solution generally known as the "primary coolant" (principally containing small variable amounts of boric acid and lithium hydroxide and substantially saturated with hydrogen) through the reactor core region of the vessel at temperatures of up to about 500.degree. F. or more and at pressures of up to about 2250 psi or more. The principal function of the primary water is to transfer heat generated by fuel assemblies in the reactor core region to one or more near-by heat exchangers for generating steam to drive turbines and thereby generating electric power. In a commercial boiling water reactor, the steam is generated in the reactor vessel itself. In other reactor designs, the core region may be cooled by liquid sodium or by gaseous coolants instead of aqueous solutions. The primary coolant also cools the internal structural members in the high temperature irradiated core region of the reactor vessels containing the fuel assemblies.
Recent ultrasonic inspections in commercial pressurized water reactors outside of the United States have indicated that the baffle/former bolts which conventionally fasten internal baffle plates (which support the fuel assemblies and guide the primary coolant through the core region) with internal former plates (which maintain the baffle plates in place in the reactor vessel cores) may be susceptible to cracking. Contemporaneous inspections of "up flow" pressurized water reactors having water (i.e., primary coolant) cooled joint designs including vertically oriented flow holes machined through the former plates, which holes interconnect with the bolt holes like those shown in U.S. Pat. No. 4,069,102, did not indicate any degradation of their baffle/former bolts. Periodic inspections of existing baffle/former bolts in commercial "down flow" commercial reactors in the United States have not indicated significant degradation which would require the nuclear power industry to replace the bolts or otherwise backfit such reactors.
Now, the nuclear power industry is considering the desirability of backfitting operating reactors in order to better cool the baffle/former bolts and thereby to provide an additional temperature margin in order to reduce their susceptibility to cracking. However, a backfit of an existing nuclear reactor vessel along the lines of the teaching of U.S. Pat. No. 4,069,102 will be very difficult because the former plates are physically located behind the baffle plates and therefore are inaccessible. Such a backfit will require either that radially directed flow holes be machined into the bolt holes of former plates from the baffle side while the vessel internals remain assembled in a submerged reactor vessel, or alternatively that the internals be removed from the reactor vessel, disassembled, and the former plates then machined remotely from the reactor vessel. Both of these backfit methods are very difficult, costly, time-consuming processes. In addition, the cost of providing replacement power will be high. Alternatively, replacement of the existing reactor internals would be very costly, although not as time-consuming.
U.S. Pat. No. 4,069,102 also proposes a bolt design having an internal bore extending down the centerline of the bolt shaft for cooling the bolt. However, such bores make the bolts difficult to ultrasonically inspect and may significantly affect the strength of the bolt.